1. Field of the Invention
The present invention relates to a metal halide lamp using a ceramic discharge tube.
2. Description of Related Art
In metal halide lamps provided with a ceramic discharge tube, there is less reactivity between a discharge tube material and enclosed metals compared to metal halide lamps having a quartz discharge tube, which has been in general use so far. Therefore, it is expected that stable lifetime characteristics can be obtained for the metal halide lamps using a ceramic discharge tube.
Conventionally, metal halide lamps having a discharge tube in which both end portions of a transparent alumina tube are closed by insulating ceramic caps or conducting caps are proposed as such metal halide lamps (see JP 62-283543 A).
Further proposed are metal halide lamps having a ceramic discharge tube having slender tube portions at both end portions of a discharge portion and having a smaller diameter than the discharge portion (see JP 6-196131 A). Electrically conductive lead wires having an electrode at their tips are inserted at the slender tube portions. The gaps between the end portions of the slender tube portions and the electrically conductive lead wires are sealed with a sealing material.
Such conventional metal halide lamps using a ceramic discharge tube utilize the high thermal resistance of the ceramic to raise a tube-wall load (lamp power per surface area of the entire discharge tube) compared to metal halide lamps having a quartz discharge tube, thereby improving the lamp efficiency. In such metal halide lamps, a technology is proposed in which the entire discharge tube is covered with a transparent protecting cylinder, thereby raising the temperature of the discharge tube so as to increase a vapor pressure of the enclosed metals and preventing an outer tube glass from breaking when the discharge tube is damaged.
In the metal halide lamps using a ceramic discharge tube, the tube-wall load of the discharge tube is raised compared to metal halide lamps having a quartz discharge tube, thereby achieving high efficiency and high color rendition. However, in order to realize high efficiency and high color rendition, it is necessary not only to raise the tube-wall load but also to keep the temperature in the discharge tube high. Accordingly, the transparent protecting cylinder or the like is used to maintain heat of the discharge tube.
On the other hand, in the metal halide lamps using a ceramic discharge tube, leaks in the discharge tube sealing portion caused by reactions of the enclosed metals with the sealing material and the lead-in wire serve as a considerable cause for dimming or dying out during the lamp lifetime. The conventional method for maintaining heat of the ceramic discharge tube of the metal halide lamps has had a problem in that, because the protecting cylinder surrounds the entire discharge tube, the temperature of the discharge tube sealing portion rises, leading to more reactivity of the enclosed metals with the sealing material and the lead-in wire. This increases the leaks in the discharge tube sealing portion during the lamp lifetime.
In addition, the metal halide lamps using a ceramic discharge tube have had a problem that the slender tube portions and the sealing portions of the discharge tube are damaged easily by a shock of dropping or the like.
It is an object of the present invention to solve the problems described above and to provide a metal halide lamp that suppresses leaks in a discharge tube sealing portion while maintaining high efficiency and high color rendition, which are features of a metal halide lamp using a ceramic discharge tube, and that is tolerant of an external shock.
In order to achieve the above-mentioned object, a metal halide lamp of the present invention includes an outer tube containing the ceramic discharge tube, and a protecting cylinder provided inside the outer tube so as to surround the discharge tube. The slender tube sealing portion of the discharge tube is exposed beyond the protecting cylinder.
Also, a metal halide lamp of the present invention includes a discharge tube of a transparent ceramic in which a discharge metal and a buffer gas are sealed, the discharge tube having electrodes at both ends thereof, including a discharge portion, slender tube portions provided at both ends of the discharge portion into which feedthroughs including the electrodes and lead-in wires are inserted, and slender tube sealing portions in which gaps between the slender tube portions and the feedthroughs are sealed with a sealing material; an outer tube containing the discharge tube; and a transparent protecting cylinder provided inside the outer tube so as to surround the discharge tube. At least one of the slender tube sealing portions is exposed beyond the protecting cylinder.
According to the present invention, heat is released appropriately from the sealing portions while keeping the temperature in the discharge tube high. Therefore, it is possible to suppress leaks in the discharge tube sealing portion while maintaining high efficiency and high color rendition.